A semiconductor photodiode is a device that provides an electric current in response to the absorption of light. Photodiodes are used in myriad applications, such as optical communications applications, sensing applications, industrial control applications, and security applications.
A photodiode provides an electric current is based upon a process known as “photo-absorption,” in which the photodiode generates a free-carrier pair (an electron and a hole) when it absorbs a particle of light (i.e., photon). To generate a free-carrier pair, an incident photon having sufficient energy transfers its energy to an electron bound within the atomic lattice of a semiconductor. The increased energy of the electron enables it to release from its bound condition (leaving behind a hole) and freely move within the atomic lattice. A bias voltage applied across the photodiode induces an electric field that accelerates generated free-carriers pairs and gives rise to a macroscopic electric current, referred to as a “photocurrent.” The magnitude of the photocurrent is proportional to the number of photons absorbed by the semiconductor.
Free-carrier pairs are also generated in the semiconductor due to absorption of thermal energy, which is directly proportional to the device temperature. Absorbed thermal energy can provide sufficient energy to a bound electron to liberate it and allow it to move freely, much in the same way that photon absorption liberates an electron. The thermal generation of free-carrier pairs is generally undesirable. It gives rise to an electrical current, referred to as “dark current.” Dark current represents noise in the output of an optical receiver that can impair detection of photocurrent, which represents the signal to be detected.
Typically, a photodiode is part of an electrical circuit that is used to detect the photocurrent. For a photodiode to produce useful information, the photocurrent must not only significantly exceed the dark current but also any unwanted noise that might be present in the electronics. Such noise can be any combinations of, but not limited to, shot noise, Johnson noise, 1/F, etc. The amount of optical power that is required to produce a photocurrent equal to the total noise in the electronics is defined as the “sensitivity” of the photodiode.
To maximize sensitivity, it is necessary to maximize the number of free-carriers generated by photo-absorption. Unfortunately, by photo-absorption alone, a maximum of one free-carrier pair can be generated for each incident photon. Fortunately, to further enhance the sensitivity of the photodiode, a third means of generating free-carrier pairs, avalanche gain, can be introduced to some semiconductor photodiodes.
A photodiode that provides avalanche gain is called an avalanche photodiode (APD). APDs can have extremely high sensitivity. In fact, an APD can be made sensitive enough to detect even a single photon. Avalanche photodiodes are so named because the motion of free-carriers through the atomic lattice of a semiconductor gives rise to additional free-carriers—much like boulders rolling down a hill during an avalanche. This free-carrier “avalanche” results from impact ionization within a region of the APD called the “multiplication region.”
Impact ionization occurs when free-carriers are accelerated to high energy by the applied electric field. As these accelerating free carriers travel through the multiplication region of the avalanche photodiode, they collide with carriers bound in the atomic lattice of the multiplication region. In each collision, some of the energy of the free carrier is absorbed by the bound carriers, freeing them from the atomic lattice. The newly freed carriers, as well as the previously generated free-carriers, are then accelerated once again by the electric field. This enables them to collide with, and liberate, more bound carriers. This process can occur very rapidly and efficiently and it is possible to generate several hundred million free-carriers from a single absorbed photon in less than one nanosecond.
The generation of free-carriers by impact ionization is parameterized by a “gain” or “multiplication” defined as the ratio of free-carriers generated with impact ionization as compared to free-carriers generated without impact ionization. Gain due to impact ionization is strongly dependent on the voltage applied across the multiplication region and applies, without prejudice, to both photocurrent and dark current.
In practice, however, it is difficult to directly measure gain. It is possible, however, to determine a bias voltage at which avalanche breakdown occurs. Avalanche breakdown is a condition in which free carriers are continuously and indefinitely generated in a self-sustaining manner in an APD without the presence of incident light. The bias voltage at which avalanche breakdown occurs is referred to as the “breakdown voltage” of the APD. Typically, the avalanche breakdown is approximated as the applied voltage that produces a certain level of multiplied, dark current.
It is well-known that the value of applied bias voltage, relative to breakdown voltage, determines the gain and/or operating mode of an APD. For linear-mode operation, an APD is biased with a bias voltage that is less than its breakdown voltage by a specified amount (an offset voltage). Gain increases exponentially as the offset voltage is reduced. For a constant offset voltage (at constant temperature), an APD operates with constant gain such that generated photocurrent is directly proportional to the intensity of light incident on the APD.
For Geiger-mode operation, an APD is biased with a bias voltage that is greater than its breakdown voltage. When biased above breakdown voltage, the gain of the APD has substantially infinite gain.
For proper operation of an APD in either linear or Geiger mode, therefore, precise and accurate knowledge of the breakdown voltage is essential. Unfortunately, the breakdown voltage for an APD, once fabricated, can vary for a number of reasons, such as temperature changes, device aging, variations in operating conditions, etc. Further, because the breakdown voltage is sensitivity to microscope changes in the properties and structure of the semiconductor in which the APD is fabricated, it is well-known that breakdown voltage will vary from device to device, wafer to wafer, and fabrication run to fabrication run due to variations in the product processes that are difficult to control.
Temperature variation affects the breakdown voltage of an APD by affecting the impact ionization rate in the semiconductor. Specifically, breakdown voltage increases with increasing operating temperature.
In addition, operation at high gains (e.g., >>10) is particularly challenging because the gain becomes increasingly sensitive to changes in bias voltage as the gain is increased. For example, when operating at high gain conditions, a change in bias voltage as small as 2% can affect a change in gain of 100% or more. In fact, an unanticipated shift in breakdown voltage or error in bias voltage can induce catastrophic failure or unacceptably large performance variation. Consequently, several challenges arise in both application and maintaining the APD bias for high gains.
First, where high-gain APD are to be employed, there requires precise knowledge of the APD breakdown voltage from device to device. This entails careful measurement of 100% of the APD devices to be used. Moreover, it requires proper configuration or calibration of the system in which the APD is to be incorporated such that the bias voltage is accurately set. While neither are unachievable processes, in practice, they are difficult and time consuming to implement and incompatible for either high-volume or low-cost processes. As a result, therefore, the breakdown voltage of an APD considered to be representative is commonly used as the bias voltage for all systems, regardless of the specific APD device used.
Second, regardless of whether the APD bias voltage can be accurately set at the begin of life of any system employing the APD, the dependence of breakdown voltage on external factors, particularly operating temperature, requires a system that continuously maintains the appropriate bias voltage over the life of the system. As a result, the value of the breakdown voltage at an operating temperature, as well as its variation over temperature must be accurately and precisely known—especially in applications requiring high APD gain. Unfortunately, individually characterizing the temperature dependence on a device-to-device basis is extremely costly and time consuming.
One commonly employed, low-cost strategy for mitigating the risks of error or variation in the bias voltage with respect to breakdown voltage is to operate APDs using a large offset voltage. This reduces any inaccuracy in the breakdown voltage relative to the increased offset voltage. Further, this also reduces the sensitivity of gain to offset voltage variation by virtue of the exponential relationship of gain to bias voltage. Still further, this mitigates the risk of the bias voltage becoming larger than the breakdown voltage, which can cause catastrophic failure for an APD-based system intended for operation in the linear-mode. However, such risk mitigation is attained at the expense of operating performance, namely sensitivity.
Another common method for enabling improved high-gain APD operation is to operatively couple the APD with a temperature compensation circuit. Compensation circuits have been developed that adjust bias voltage in response to changes in breakdown voltage due to temperature changes. This requires an ability to accurately detect a shift in the breakdown voltage of the APD, however. During normal operation, it is not typically possible to directly measure the breakdown voltage of the APD. In lieu of direct measurement, therefore, the temperature of the APD is commonly used to infer a value for the breakdown voltage. The temperature of the APD is estimated from the temperature of the carrier (monitored using a temperature sensitive element, such as a thermistor, thermocouple, Schottky diode, etc.) on which the APD is disposed. This temperature is then used, along with a predetermined model of the relationship between temperature and breakdown voltage, to calculate a value for the breakdown voltage of the APD. Such an approach offers no relief for breakdown voltage variation due to changing operating conditions, however. The effectiveness of such a method is also limited by the accuracy of the predetermine model of the breakdown voltage with respect to temperature.